Process for surface treatment of lead and its alloys

ABSTRACT

THIS IS A METHOD FOR THE SURFACE TREATMENT OF LEAD AND ITS ALLOYS WHICH RESULTS IN AN IMPROVED JOINT SUBSEQUENTLY FORMED BETWEEN THE METAL AND ORGANIC MATERIALS AND COMPRISES TREATMENT IN AN AQUEOUS SOLUTION CONTAINING DICHROMATE IONS AND HYDROFLUORIC, FLUOBORIC, OR ACETIC ACID. DESCRIBED EMBODIMENTS OF THE SURFACE TREATMENT INCLUDE USE OF THE SOLUTION BOTH ASS A LIQUID AS A GEL.

y 7. H. N. VAZIRANI 3,591,432

PROCESS FOR SURFACE TREATMENT OF LEAD AND ITS ALLOYS Filed May 8, 1968[/5 N TOR By H. AZV VAZ/RAN/ ATTVRNEV United States Patent US. Cl. 156-39 Claims ABSTRACT OF THE DISCLOSURE This is a method for the surfacetreatment of lead and its alloys which results in an improved jointsubsequently formed between the metal and organic materials andcomprises treatment in an aqueous solution containing dichromate ionsand hydrofluoric, fluoboric, or acetic acid.

Described embodiments of the surface treatment include use of thesolution both as a liquid and as a gel.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to the surface treatment of lead and its alloys by the use of asolution containing dichromate ions and hydrofluoric, fiuoboric oracetic acid in order to improve joints subsequently formed between suchsurface and organic materials, and also relates to the joined product.

(2) Description of the prior art Joining lead and its alloys to organicmaterials is desirable in many cases, as for example, in the formationof laminates to be used in the manufacture of coaxial cable and in therepair of cable whose lead sheathing has devel oped aging cracks.Corosion products ordinarily present on the surface of the metalinterfere with such joining and have in the past been removed bothchemically, as for eX- ample, by fluoboric acid treatment, andmechanically, as for example, by wire brush abrading Mechanical abrasionis generally undesirable because it is wasteful of the soft lead surfaceand produces a rough texture which gives rise to localized stresses insubsequently formed joints when placed under stress.

Chemical cleaning is less damaging to the surface of the metal. However,the clean surface is subject to further corrosion and it is oftenimpractical to form joints with other materials before substantialreformation of the corrosion products has taken place. In addition,joints subsequently formed, even with a substantially clean metalsurface, often degrade with time resulting in eventual joint failures.Depending on th temperature, stress and humidity levels, such failuresmay occur after only a few days.

Attempts to replace the metal corrosion products by a chemicallyproduced oxide layer in order to prevent further corrosion and provide asurface suitable for joining have generally been unsuccessful because ofthe mechanical weakness of the lead oxides formed.

SUMMARY OF THE INVENTION This invention is essentially a joiningtechnique in which the surface of lead and its alloys are treated with asolution containing dichromate ions and hydrofluoric, fluoboric oracetic acid prior to joining. Such treatment results in removal ofcorrosion products and their replacement by a mechanically strongprotective layer thus enabling joints to be formed which exhibitexcellent initial strengths and which resist degradation with time.

The invention encompasses treatments with the solution either as aliquid, preferred when optimum improvement of joints is desired, or as agel, preferred when use of a liquid would be inconvenient.

Since the surface to be treated ordinarily contains contaminants such asdirt, grease and corrosion products, it is usually necessary to removethese prior to the surface treatment and to aid the practitioner,exemplary procedures are briefly described.

Furthermore, since the treatment is essentially one to prepare thesurface of lead and its alloys for joining to an organic material,joints to dissimilar metals as well as to similar metals and joints tononmetals, all by means of an organic material, are contemplated.

BRIEF DESCRIPTION OF THE DRAWING The figure is a cross-sectional view ofa laminate formed according to the inventive process.

DETAILED DESCRIPTION This process applies to lead and any of its alloyscontaining at least lead, and has as its principal object the removal ofdirt and corrosion products from the metal surafce and replacement witha chemically produced layer. Since an object of the surface treatment isto remove oxide scale, any advantage to be gained by preliminarycleaning of the metal surface will ordinarily reside in the reduced rateof contamination of the treating solution, replacement of which may beboth costly and time consuming.

Preliminary cleaning is by methods well known in the art. Thus, thefollowing description of preliminary cleaning is intended to beexemplary and not limiting.

Preliminary cleaning is generally divided into degreasing descaling. Forexample, degreasing is generally effective in removing only oils andgrease and is ineffective in removing corrosion products such asnaturally formed oxide scale. Descaling, which may be either mechanical,will, however, generally remove substantially all of the surfacecontamination. However, descaling chemically without first degreasingmay result in rapid contamination of the solution.

Removal of oils and grease may be accomplished by the use of organicsolvents such as alcohols, ketones and chlorinated solvents such astrichlorethylene and perchlorethylene.

Descaling is usually accomplished by the use of acid solutions or bymechanical abrasion. The particular method chosen for descaling willdepend on the thickness, composition and character of the scale, whichdepends upon the composition of the metal and upon its history.

A more complete description of degreasing and descaling methods may befound in Protective Coatings for Metals, third edition, AmericanChemical Society, Monograph 163 by R. M. Burns, and W. W. Bradley, pages27 to 54, Reinhold (1967).

Once the removal of surface contamination has been effected, the cleanmetal surface should be either treated promptly or stored undernoncorrosive conditions until treatment in order to realize anyadvantage gained, since exposure of the clean surface to a nonprotectiveatmosphere will soon result in reformation of corrosion products.

The concentration of dichromate ions in the treating solution is notcritical and may range from 0.001% by weight to saturation. The ions maybe introduced in combination with the Group I alkali metals, lithium,sodium, potassium, rubidium and cesium or the Group II alkaline earthmetals; beryllium, magnesium, calcium, strontium and barium. The acid inthe solution may be hydrofluoric, fluoboric, or acetic.

The concentration of acid is likewise not critical, and may range from0.1% by volume to saturation. Below the respective lower limits ofconcentration for dichromate 3 ions and acid, a significant improvementin resulting joints will generally not be observed.

Hydrofluoric and fluoboric acids are preferred over acetic acid, sincetheir presence in the treating solution results in a strong etchingaction, which is beneficial in the obtaining of optimum joints. Althoughhydrofluoric acid is unstable when present in amounts greater than about50% by volume, it is preferred since a high acid concentration isunnecessary to the obtaining of satisfactory joints, and sincehydrofluoric acid is cheaper than fluoboric acid. Acetic acid, whileless effective than the stronger acids, may be preferred where it isdesired to avoid handling corrosive fluoride-containing solutions.

For use of the treating solution as a liquid, it may be convenient toform the acid by adding a Group I or Group II salt of fluoride oracetate, such as sodium fluoride or sodium acetate. Such a solutionwould, however, generally not give as good results as would a morestrongly acidic solution.

A preferred treating solution for use as a liquid is one containing from4% to 5% by volume of a 49% by weight solution of hydrofluoric acid andfrom 3% to 5% by weight of dichromate ions. Such a solution has beenfound to give a thin, strong layer suitable for joining.

Exceeding these preferred ranges in the direction of too muchhydrofluoric acid or too little dichromate will result in a layer toothin for optimum improvement of joints, while too little acid or toomuch dichromate will result in a layer too thick for optimumimprovement.

Contact of the metal surface with the liquid for at least one secondwill ordinarily result in a noticeable improvement in the subsequentlyformed joints. However, treatment times of from seconds to 60 secondsare preferred in order to insure complete reaction.

Where it is desired to treat the metal surface under conditions whichmake inconvenient the handling of liquid solutions, a gel of thetreating composition may be formed. Such may be accomplished by adding agelling agent such as sodium silicate gradually, with agitation, to thetreating solution until a gel of the desired consistency is attained.Alternatively, acid may be added to a solution containing sodiumdichromate and the gelling agent until the gel is formed.

It is preferred to use an inorganic gelling agent such as sodiumsilicate, since organic gelling agents are generally unstable in acidsolutions. Where acetic acid is used together with sodium silicate, thegel formed may tend to separate after standing for a few hours.Agitation will cause substantial reformation of the gel, however. A morestable gel may be formed using finely divided silica as the gellingagent. It will be appreciated however, that finely divided silica willnot form a gel of a solution containing hydrofluoric or fluoboric acid,since it is soluble in the acids.

A preferred gel composition is one which initially contains from 1 to10% by weight each of dichromate ions and acid, and having a pH of from1 to 3.

Momentary contact of the gel with the metal results in a noticeableimprovement in the subsequently formed joints. However, it is preferredto leave the gel in situ for from one-half to five minutes, in order toinsure complete reaction. After removal by wiping, rinsing or otherconvenient method, the metal is ready for joining.

Since a certain amount of acid etching is required for successfulsurface treatment, the liquid is preferred in this regard, becauseaddition of a gelling agent will usually result in formation of a weakacid. For example, adding sodium silicate to a solution containinghydrofluoric acid results in the formation of silicic acid.

The following examples compare adhesive joint strengths obtained byvarious embodiments of the inventive treatment and by other treatments.

EXAMPLE 1 Four sets of T-peel joints were made from specimens of leadcontaining 1% antimony and having dimensions of about one inch in widthand twenty mils in thickness and an acrylic acid-ethylene copolymeradhesive as per ASTM procedure D1876-61T. Treatment prior to joining wasas follows: All of the specimens were vapor degreased and grit blastedwith alumina powder. Set 3 was treated for ten seconds in a solutioncontaining 3% by weight of sodium dichromate and 10% by volume of 49%hydrofluoric acid. They were then rinsed with distilled water and dried.Set 4 was treated as was Set 3 except that the solution additionallycontained 5% by volume of a 98% sulphuric acid solution. The results areshown in Table 1 as the average T-peel strength in inch-pounds per inchwidth for each set of specimens.

As may be seen from the results in Table 1, the presence of sulfuricacid in the treating solution interferes substantially with theobtaining of optimum results. Other additives, such as various salts,will not significantly enhance the final result, and may interfere withthe formation of a surface suitable for joining when present in largeamounts. In general, additives which may interact with lead ions toproduce insoluble products will interfere with the obtaining of optimumresults when present in amounts greater than about 0.1% total. Additiveswhich cause interference with the etching action of the acid, such aswas already noted for some gelling agents, will of course also beundesirable for the obtaining of optimum results.

EXAMPLE 2 Four sets of T-peel joints were made as in Example 1.Treatment prior to joining was as follows: All of the specimens werevapor degreased. Sets 3 and 4 were brushed with a paste formed by addinga 20% by weight solution of sodium silicate, with mixing, to a solutioncontaining 10% by weight each of hydrofluoric acid and sodiumdichromate, until a gel was formed. The paste was wiped from thespecimens after about two minutes. The results are shown in Table 2 asthe average T-peel strength in inch-pounds per inch width after one dayof aging in ambient and after 14 cycles of two cycles a day each betweentemperatures of and 120 F., and humidities of 80 and 100%.

Two sets of T-peel joints were made as in Example 1. Treatment prior tojoining was as follows: All of the specimens were vapor degreased. Set 2was treated as were Sets 3 and 4 in Example 2 with a paste formed as inExample 2, except that 10% by weight of acetic acid was substituted forhydrofluoric acid in the solution. Results are shown in Table 3 as theaverage T-peel strength in inch-pounds per inch width.

TABLE 3 T-peel strength:

As may be seen from the examples, durable joints under static load areobtainable by use of the inventive solution prior to joining, whetherthe solution be in the liquid or gel form.

Referring now to the drawing, there is shown a bonded product preparedin accordance with the inventive process. Shown in the figure is a leador lead alloy body I joined to an organic body 2. An illustrativeembodiment of the joined product is lead in the form of cable sheathing,coated with an organic coating material such as an alkyd or modifiedalkyd, an acrylic, an epoxy, a polyester or a urethane, subsequent tothe described surface treatment.

Other embodiments of the invention which basically rely on the teachingsof the invention as described herein are to be considered as within thescope of the invention and the appended claims.

What is claimed is:

1. A method for preparing a surface containing at least 75 percent byweight lead for adhesive joining to an organic material characterized inthat said surface preparation consists essentially of contacting saidsurface with a solution formed from initial ingredients comprising: anaqueous solution containing from 3 to percent by weight of dichromateions, and from 4 to 5 percent by volume of at least one member selectedfrom the group consisting of hydrofluoric acid and fiuoboric acid.

2. The method of claim 1 in which contacting said surface with saidsolution is carried out for a time of at least ten seconds.

3. The method of claim 1 in which the step of contacting said surfacewith said solution is preceded by descaling.

4. The method of claim 1 in which dichromate ions are introduced intosaid solution as a compound of an element selected from Groups I and IIof the Periodic Table.

5. The method of claim 1 in which the step of contacting said surfacewith said solution is followed by adhesive joining said surface to anorganic material selected from the group consisting of epoxies, alkyds,acrylics, polyesters and urethanes.

6. The product produced by the method of claim 1.

7. A method for preparing a surface containing at least percent byweight lead for adhesive joining to an organic material characterized inthat said surface preparation consists essentially of contacting saidsurface with a gel formed from initial ingredients comprising; anaqueous solution containing from 1 to 10 percent by weight of dichromateions and from 1 to 10 percent by volume of hydrofluoric acid andadditionally contains sutficient gelling agent to bring the pH of thesolution to a value between one and three.

8. The method of claim 7 in which the gel is formed by the addition ofsodium silicate to said solution.

9. The method of claim 7 in which said surface is contacted with saidgel for a time of from one-half to five minutes.

References Cited UNITED STATES PATENTS 132,746 11/1872 Berry l48-6.21,539,993 6/1925 Conley et al. 1486.2X 2,106,904- 2/1938 Wilhelm l486.22,868,705 1/1959 Baier et a1. 1486.l6X 3,366,514 1/1968 Chadha et al.14862 FOREIGN PATENTS 606,022 9/1960 Canada 1486.2

JOHN T. GOOLKASIAN, Primary Examiner J. C. GIL, Assistant Examiner US.Cl. X.R.

ll749, 71; 1343, 41; 1486.2; 156l8, 20, 316; 16l-2l3; 25279.3

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. '3 591,M32 Dated July 6, lQYl Inve t Hargovind N Vazirani It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column line 1 29 change "surface" to surfaces 1 #2 insert a period atthe end of the line 1 5 L change "th" to -the 1 6H change "surface" to-surfaces 2 22 change "surafce" to -surface i 2 33 before "descaling"insert and- 2 36 after "mechanical, insert or chemical,-

2 51 change "Protective Coatings for to -Protective Coatings for 2 52change "Metals" to -Metals- 2 65 after "metals" delete and insert 5 18after "comprising" delete and insert Signed and sealed this 18th day ofJanuary 1972.

(SEAL) Attest:

EDWARD MJ LETCHER JR. ROBERT GOTTSCHALK Attastin", Officer ActingCommissioner of Patents FORM PO-1 LJSCOMM-DC scam-ps9 LLS GOVERNMENTPRINTING OFFICE I959 OJ56 33l

